This problem arises with all fine powders, as I was saying, but we're going to continue this post by focusing on flour.
Sifting
The purpose of sifting is simply to ensure that you add a fine flour powder to your recipe, which will incorporate well with the other ingredients, without making lumps.
Unfortunately, flour always ends up clumping together, mainly due to humidity, and forms little balls, often invisible because they're drowned in the mass.
It's not a question of the product itself - even when agglomerated into pellets, it's still good flour. It's more a question of texture: pellets don't mix as well as fine powder, and they can cause lumps, or remain in a ball in your dough.
What's more, they are rarely visible: you pour flour into your preparation, everything seems normal, you have a nice fine powder, but the pellets may be there.
The easiest way to get rid of them is to sift the flour.
Look at the photo of T55 flour, before sifting...
Once there are only pellets left in the bottom of your sieve, simply press down to reduce them to powder, sieve again, and continue your recipe undisturbed.
Is this really necessary?
You might think, well, it's flour, so it's still mostly fine powder, and I put it all in a mixer or whisk it carefully, so there's no need to sift.
Of course, in absolute terms, this is true, but it's still better to sift, because your dough will mix faster and better, and it's also an opportunity to eliminate any squatter insects that might be present in the flour, especially in organic flour.
Do I have to sift everything?
A professional pastry chef would certainly say yes, but I think that if you limit yourself to the flours and powdered sugar, you'll already be in very good shape for the rest of your recipe.
Special mention should be made of the chestnut flour and powdered sugar (see photo), which clump together very, very easily, as you'll soon see when you open the packet or box.
To sum up: it's best to sift all the fine powders that go into your recipe, as this is always better for the texture of the mixture you're about to make, and to avoid lumps.
And it's not Édith who will contradict me ;-)